1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device including a ferroelectric capacitor.
2. Description of the Related Art
In recent years and continuing, attention is being directed to a ferroelectric memory employing a ferroelectric capacitor as a high-speed low-power nonvolatile memory, and research and development are being vigorously conducted relating to this field. It is noted that a planar type ferroelectric capacitor has conventionally been used in a ferroelectric memory capacitor as is described above. However, use of a ferroelectric capacitor having a stacked capacitor structure is being contemplated to realize high integration of ferroelectric capacitors.
It is noted that a ferroelectric film implemented in a stacked capacitor needs to have good step coverage, and be made of high density crystal with a low leak current in order to realize a three-dimensional capacitor structure.
Accordingly, in forming a ferroelectric film implemented in a stacked capacitor, the MOCVD (Metal Organic Chemical Vapor Deposition) method that is capable of realizing good step coverage and a high density crystal is preferably used over other methods such as the sol-gel method or the sputtering method. In turn, the MOCVD method is presently being regarded as the next generation film deposition method.
FIGS. 1A˜1G illustrate process steps for fabricating a semiconductor device including a ferroelectric memory having a stacked capacitor structure.
Referring to FIG. 1A, a well region 103 corresponding to an impurity dispersion region that is isolated by an isolation film 102 is formed on a substrate 101 that is made of Si. In the well region 103, low concentration impurity dispersion regions 106 and high concentration impurity dispersion regions 107 are formed in a manner such that the low concentration impurity dispersion regions 106 cover the high concentration impurity dispersion regions 107.
Also, a channel region 104 is formed in between two low concentration impurity dispersion regions 106 formed within the well region 103. A gate electrode 108 that may be made of polysilicon, for example, is formed on the upper side of the channel region 104 via a gate dielectric film 105. A side wall insulating film 110 is formed at the side wall of the gate electrode 108, and an insulating film 109 is formed on the upper portion of the gate electrode 108. Further, an insulating film 111 is formed to cover the side wall insulating film 110, the insulating film 109, and the high concentration impurity dispersion regions 107, and an interlayer insulating film 112 is formed to cover the insulating film 111. In this way, a MOS transistor 200 is formed.
As is shown in FIG. 1A, two MOS transistors 200 may be formed within the well region 103, for example. Also, contact plugs 113 covered by barrier films 113a that are electrically connected to the high concentration impurity dispersion regions 107 are formed between the two MOS transistors 200 and between the MOS transistor and the isolation film 102 within the interlayer insulating film 112.
Next, in the process illustrated by FIG. 1B, an Ir film 114a for realizing a lower electrode of the capacitor is formed on top of the interlayer insulating film 112, and then, a Pb(Zrx, Ti1-x)O3 film (PZT film) 115a is formed on the Ir film 114a through the MOCVD (Metal Organic Chemical Vapor Deposition) method. In forming the PZT film 115a through the MOCVD method, for example, organic metal gas and oxide gas as source gases may be supplied to the substrate 101 that is thermally processed so that the organic metal gas may be thermally decomposed and an oxidative reaction may occur between the organic metal gas and the oxide gas. In this way, the PZT film 115a may be formed on the Ir film 114a. 
It is noted that source gas including Pb, source gas including Zr, and source gas including Ti, for example, may be used as the organic metal gas, and oxygen may be used as the oxide gas.
Then, an IrOx film 116a for realizing an upper electrode of the capacitor is formed on the PZT film 115a. 
Next, in the process as is illustrated in FIG. 1C, an etching process is conducted on the IrOx film 116a, the PZT film 115a and the Ir film 114a to form a ferroelectric capacitor 130 including a lower electrode 114 that is made of Ir, a ferroelectric film 115 that is made of PZT, and an upper electrode that is made of IrOx.
Next, in the process as is illustrated in FIG. 1D, a protective film 117 is formed to cover the ferroelectric capacitor 130 and the interlayer insulating film 112. Then, in the process as is illustrated in FIG. 1E, an interlayer film 118 is formed to cover the protective film 117.
Next, in the process as is illustrated in FIG. 1F, a contact hole is formed at the interlayer insulating film 118, and a contact plug 120 covered by a barrier film 12a is formed to be electrically connected to the contact plug 113 provided between the two MOS transistors 200.
Next, in the process as is illustrated in FIG. 1G, a contact hole that comes into contact with the upper electrode 116 is formed at the interlayer insulating film 118, and a wiring layer including a wiring portion 119 that is covered by barrier layers 119a and 119b is formed to be electrically connected to the upper electrode 116 and the contact plug 120. Then, by forming a multilayer wiring structure that is connected to the above wiring layer, a semiconductor device including a ferroelectric memory is formed.
It is noted that the characteristics of the ferroelectric capacitor 130 that is formed in the above-described manner are largely dependent on the ferroelectric characteristics of the ferroelectric film 115. The ferroelectric characteristics of the ferroelectric film 115 depend on the orientation of the PZT crystal of the ferroelectric film 115, and the ferroelectricity of the ferroelectric film 115 may be maximized (i.e., a maximum switching charge Qsw may be obtained) when the PZT crystal has a (001) orientation.
On the other hand, ferroelectricity may not be achieved when the PZT crystal is (100) oriented. Generally, the PZT crystal of the ferroelectric film 115 belongs to the tetragonal crystal system, and thereby, the lattice constant of the c axis direction is different from the lattice constants of the a axis direction and the b axis direction. However, in practice, the difference in the lattice constants is very small so that when attempts are made to direct the PZT crystal in the (001) orientation, the PZT crystal may equally be directed in the (100) orientation with the same probability. Accordingly, a technique is proposed for increasing the proportion of the (111) oriented PZT crystals. Although this leads to degradation of the ferroelectricity of the ferroelectric film, the overall intrinsic polarization of the ferroelectric film may be increased, good imprint characteristics may be achieved, and reliability in the ferroelectric memory may be improved according to this technique.
It is known that in order to increase the proportion of (111) oriented PZT crystals, the temperature of the film deposition process for forming the PZT film must be set to at least 600° C. (e.g., see Horii et al., IEDM Technical Digest, 2002, p. 529).
FIG. 2 is a graph representing X ray analysis profiles of PZT films formed on the Ir film at different film deposition temperatures using the MOCVD method as is described in relation to FIG. 1B. In FIG. 2, results of forming the PZT film at temperatures of 450° C., 500° C., 550° C., 580° C., and 620° C. are represented as experiments E1, E2, E3, E4, and E5, respectively. In FIG. 2, peak P1 represents a (100) orientation in the PZT film, peak P2 represents a (101) orientation in the PZT film, and peak P3 represents a (111) orientation in the PZT film.
It is noted that peak Ps and peak Pi of FIG. 2 represent the (111) orientations of Si and Ir, respectively.
First, in the experiment E1 of FIG. 2, neither the peak P1 representing the (100) orientation in the PZT film, the peak P2 representing the (101) orientation in the PZT film, nor the peak P3 representing the (111) orientation in the PZT film can be seen. Accordingly, it may be understood that when the film deposition temperature is below 500° C. (e.g., 450° C.), the PZT film is formed into a non-crystalline state.
Next, in the experiments E2˜E4, although the peak P1 representing the (100) orientation in the PZT film and the peak P2 representing the (101) orientation in the PZT film can be seen, the peak P3 representing the (111) orientation in the PZT film cannot be found. Thus, it may be understood that in a case where the film deposition temperature is at least 500° C. but less than 600° C., the crystallization of the PZT is in progress but the (111) orientation is still not formed.
Next, referring to experiment E5 of FIG. 2, when the film deposition temperature for forming the PZT film is greater than or equal to 600° C. (e.g., 620° C.), the (111) orientation may be found in the PZT film. As can be appreciated from the above experiment results, in the MOCVD method, the (111) orientation ratio in the PZT film may be increased when the film deposition temperature for forming the PZT film is greater than or equal to 600° C.
On the other hand, according to research conducted on which the present invention is based, it is known that when the PZT film is formed at a film deposition temperature of 600° C. or higher, the adhesion rate of the organic metal source gas to the substrate decreases, and thereby, the deposition speed of the PZT film decreases.
FIG. 3 is a graph representing the substrate adhesion rates of organic metal source gases for forming the PZT film in relation to the film deposition temperature. In FIG. 3, the adhesion rates of a source gas including Pb, a source gas including Zr, and a source gas including Ti are shown as examples of organic metal source gases for forming the PZT film.
As is shown in FIG. 3, for each of the source gas including Pb, the source gas including Zr, and the source gas including Ti, the adhesion rate with respect to the substrate decreases as the film deposition temperature is increased. For example, when the film deposition temperature is at 620° C., the adhesion rate with respect to the substrate is decreased compared to the case in which the film deposition temperature is between the range of 500˜550° C.
This effect is a result of the decomposition of the metal organic gas in the vapor phase which reduces the amount of the metal organic gas being adhered to the substrate. In turn, the amount of decomposition products of the source gas such as particles generated in the vapor phase is increased; that is, the amount of impurities generated in the film deposition process is increased. When such impurities are included in the PZT film, local degradation of ferroelectric characteristics in the PZT film may occur, and the switching charge at bits including particles may be degraded thereby leading to decrease in the yield.
Also, in forming the PZT film, when the film deposition temperature is greater than or equal to 600° C., Pb, which has a high vapor pressure, is particularly prone to separation, and with the separation of Pb, separation of oxygen occurs. Thus, crystal defects such as a Pb deficit and/or an O (oxygen) deficit within the PZT film may be increased.
When the amount of crystal defects is increased, the leak current of the PZT film is increased, and fatigue characteristics of the ferroelectric capacitor using the PZT film are degraded.